1. Field of the Invention
This invention relates to an apparatus employing a single antenna to transmit and receive, at low loss and without mutual interference, signals in different frequency bands, such as mobile telephone signals and radio broadcasting.
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional transmission/reception apparatus 50 for a mobile telephone. For mounting a mobile telephone on an automobile, the antenna provided for the reception of radio broadcasts is shared because its transmission frequency band f1 is different from the frequency band f2 of the radio broadcasts. In order to share the antenna in this way, the signal line of the mobile telephone is connected with the signal line of the radio set. Therefore, when a radio broadcast is received while using the mobile telephone, the so-called beat noise is mixed in the sound reproduced by the radio set. To prevent the generation of such beat noise, the elements shown in FIG. 1 have been used hitherto.
The frequency band f2 of radio broadcasts is, in AM broadcasts, frequency band f2a, that is, 500 to 1620 kHz, and, in FM broadcasts, frequency band f2b, that is, 76 to 90 MHz. In the mobile telephone, on the other hand, for radio communication with the ground station connected with the telephone line, a frequency band f1a of 870 to 890 MHz is used in receiving, and a frequency band f1b of 920 to 940 MHz is used in sending. The prior art shown in FIG. 1 makes use of such a difference in frequency bands.
In other words, as shown in FIG. 1, a radio set 51 is connected to an antenna 53 by way of a low pass filter 52, and the mobile telephone 54 is connected to the antenna 53 by way of a high pass filter 55. The signal line connected to the mobile telephone 54 is joined to the signal line connected to the radio set 51. During use of the mobile telephone 54, since the frequency band f1 of the signals transmitted or received by the mobile telephone 54 is relatively high, the radio set 51 will not generate beat noise by the interference with the signal in the frequency band f2 used in the mobile telephone 54 owing to the low pass filter 52.
The equivalent circuit of the antenna 53 and the typical circuit composition of the low pass filter 52 are shown in FIG. 2. A capacitor C11 is connected in series to a signal source 56, and coils L11 and L12 are connected in series to this capacitor C11. The contact point 57 of coils L11 and L12 is grounded by way of another capacitor C12.
The relation between voltage V11 generated in signal source 56 and output voltage V12 of the low pass filter 52 due to electrostatic capacity of capacitors C11 and C12 is as follows: ##EQU1## That is, in the low pass filter 52, since the capacitor C12 is provided between the signal line and the ground, the output voltage V12 of the low pass filter 52 unfavorably becomes smaller than the generated voltage V11 in the signal source 56. In eq. 1, since radio broadcasts are to be received, the attenuation of signals by coils L11, L12 is assumed to be sufficiently small.
FIG. 3 is an equivalent circuit diagram in the frequency band f2a of AM broadcast of an antenna 61 and a cable 62 in a different prior art device.
In a car-mounted radio set, it will be very convenient if FM radio signals, AM radio signals, and mobile telephone signals can be received by one antenna. In an antenna which is extended or retracted by a motor or the like, a signal cable cannot be attached to the lower end of the antenna, and it is difficult to shorten the signal cable. Accordingly, the cable capacity of the signal cable increases, and the impedance derived from the cable capacity becomes high. In particular, in radio signals of a relatively low frequency band such as AM radio signals, the effect of cable capacity becomes larger. Therefore, in a car-mounted antenna, signals in a wide frequency band must be sent out to the radio set while suppressing the loss by the signal cable.
The antenna 61 can be expressed in terms of antenna effective capacity Ce and antenna reactive capacity Ca, and the AM radio signals received by this antenna 61 can be expressed in terms of an alternating-current power source V21. The cable 62 can be shown as a line l11 between terminals A1 and B1, and this line l11 is grounded by way of cable capacity Cb. The signal at the terminal B1 is fed into a radio set. The voltage V22 at this terminal B1 is expressed as follows: ##EQU2## As expressed in eq. 2, supposing that the cable capacity Cb is large, the gain of the AM radio signals of relatively low frequency received by the antenna 61 is lowered so that the cable capacity Cb makes the receiving sensitivity and the ratio of signal to noise (S/N ratio) drop.
To prevent such a drop in receiving sensitivity and S/N ratio, an amplifier (not shown) is placed between the antenna 61 and the cable 62, that is, at the position of terminal A1, so that the receiving sensitivity and S/N ratio are improved. In such an antenna, since active elements are used, they give rise to an increase in cost, and also involve other problems such as maintaining a circuit characteristic of suppressing only the distortion of signals at the time of input of a strong electric field. In addition, new problems may be also experienced, such as loss due to impedance conversion in the amplifier, and insufficient matching of impedance.